The potential energy surfaces for the reactions of fused tricyclic dimetallenes that feature a highly strained E=E double bond, Rea-E=E, where E = C, Si, Ge, Sn, and Pb, were studied using density functional theory (B3LYP/LANL2DZ). Three types of chemical reactions (i.e., a self-isomerization reaction, a [2 + 2] cycloaddition with a ketone and a methanol 1,2-addition reaction) were used to determine the reactivity of the Rea-E=E molecules. The theoretical findings reveal that the smaller the singlet triplet splitting of the Rea-E=E, the lower are its activation barriers and, in turn, the more rapid are its chemical reactions with other chemical molecules. Theoretical observations suggest that the relative reactivity increases in the following order: C=C << Si=Si < Ge=Ge < Sn=Sn < Pb=Pb. Namely, the smaller the atomic weight of the group 14 atom (E), the smaller is the atomic radius of E and the more stable is its fused tricyclic Rea-E=E to chemical reaction. It is thus predicted that the fused tricyclic Rea-C=C and Rea-Si=Si molecules should be stable and readily synthesized and isolated at room temperature. The computational results show good agreement with the available experimental observations. The theoretical results obtained from this work allow a number of predictions to be made.